Null file path during an LTO process means:
1.) The file(s) has/have been changed/altered/offline while the backup is running so the backup job will not be able to find the file(s).
2.) The file(s)/folder(s) contain illegal characters that the Windows OS are not accepting and must be altered for the OS to process the backup
3.) The file(s)/folders(s) are too long in length or are too deep in file path that it considers the character count too long
Windows Server 2012: 255 for both files and folders combined for the entire path.
Windows Server 2016: 255 for individual files/folders. Total for path has an extremely high limit.